Ah!, Hadn't read the Pratchett thing. Many thanks- I had a chat with him about 3 years ago. Very interesting guy.
Still think you are flogging a dead horse with the total reaction thing. Always best to answer in a context people already understand,and not to appear too clever .
Remember a question on Hawk Groundschool exam.
'Does pressure increase or decrease in a convergent nozzle?'
Me: "Sir, it depends. Is the flow subsonic or supersonic?"
QFI "If you're smart enough to ask that, then you're smart enough to know the answer they want"
Wound my neck in after that!